Rollback existing nested transacvtion before restarting
Rollback existing nested transacvtion before restarting

SQLAlchemy 1.4 will be adding stricter rules to Connection objects
such that a new transaction cannot be started while an old one hasn't
been cleared.   Ensure this particular test fixture clears
the "savepoint" transaction first before starting a new one.

References: https://github.com/sqlalchemy/sqlalchemy/issues/4712
Change-Id: Idcb616fd3add4d58f22b91865cec8a54fe492093

openstack/oslo.db,openstack/oslo.db